Faults on underground electrical power distribution systems are of increasing concern as the source of customer supply interruptions, particularly in view of the existing and foreseen installations of underground primary cable. Utility equipment and practices presently used to locate faults often cause time delays in restoring supply, can be a frequent cause of further system damage, and can involve significant costs during fault location.
While the currently available fault location equipment is technically adequate to locate all faults on distribution cables, it is very inefficient, especially if the distribution transformers are left connected, Usually a utility must maintain a wide range of fault locating equipment to be able to handle the different combinations of fault and cable types. Also it is desirable that the fault locating equipment operate without disconnection of distribution transformers and/or loads, in order to avoid customer downtime and practical difficulties. This, however, requires that the equipment be capable of discriminating between transformers and/or load characteristics and a fault. It is further desirable that, if current is allowed to flow in the fault during the detection interval, the current should not adversely effect system components or apparatus. If a fault current flows, it: (a) should not cause fuse blowing nor reach minimum-melt level of fuses; (b) should not impose detrimental duty on any system apparatus such as switches, reclosers, breakers, transformers, surge arrestors, etc; and (c) duration should be minimized.
In U.S. Pat. No. 4,227,145 there is disclosed an apparatus for detecting faults in electric cables, including (a) a solid state switch unit connected in series with the cable under test and the normal source of voltage supply of the cable when it is under load, (b) a metering unit having a resistor connected in series with the switch unit and a peak detector for measuring the voltage developed across the resistor, and (c) a trigger circuit having a manually operated trigger switch which causes the switch unit to be conductive for a preset period. This period is said to be long enough to allow the metering unit to indicate the size of the test current flowing through the resistor but not long enough in time for such current to damage the apparatus, the cable and its normal voltage supply. A fuse is connected in series with the solid state switch unit and is rated to blow before any damage occurs in such switch unit. The apparatus is employed to locate faults in low voltage distribution cables.
The apparatus in the above described U.S. Pat. No. 4,227,145 has the same disadvantages of some other known devices including its limitation to use in low voltage distribution cables and its operation to limit only the period of time during which the current flows and not also the magnitude of such current. Also, the patented apparatus requires auxiliary power for its operation and thus presents some limitation where power sources are not readily available in the field.
It is an object of the present invention to provide a fault locator for electrical power distribution systems which can be used at essentially all primary distribution high voltage levels.
It is another object to provide a fault locator which is readily adapted for use with different fuse ratings.
It is another object to provide a fault locator which limits both the magnitude and time duration of the current so as not to adversely effect system components.
It is a further object to provide a fault locator which requires no auxiliary power to operate to thereby facilitate field testing.